1. Field of the Invention
This invention relates to the field of capacitive transducers and associated electronic circuitry, and relates in particular, but not exclusively, to an apparatus and method for testing capacitive transducers and/or their associated electronic circuitry, for example micro-electrical-mechanical systems (MEMS) capacitive transducers and their associated circuitry.
2. Description of the Related Art
Consumer electronics devices are continually getting smaller and, with advances in technology, are gaining ever increasing performance and functionality. This is clearly evident in the technology used in consumer electronic products such as, for example, mobile phones, laptop computers, MP3 players and personal digital assistants (PDAs). Requirements of the mobile phone industry, for example, are driving components to become smaller with higher functionality and reduced cost. For example, some mobile phones now require multiple microphones for noise cancelling, or accelerometers to allow inertial navigation, while maintaining or reducing the small form factor and aiming at a similar total cost to previous generation phones.
This has encouraged the emergence of miniature transducers. For example, in respect to speech applications, initially electret microphones were used to capture speech, but more recently micro-electrical-mechanical (MEMS) transducers have been introduced. MEMS transducers may be used in a variety of applications including, but not limited to, pressure sensing, ultrasonic scanning, acceleration monitoring and signal generation. Traditionally such MEMS transducers are capacitive transducers some of which comprise one or more membranes with electrodes for read-out/drive deposited on the membranes and/or a substrate. Relative movement of these electrodes modulates the capacitance between them, which then has to be detected by associated electronic circuitry such as sensitive electronic amplifiers.
FIG. 1 illustrates a schematic diagram of a MEMS device 99 comprising a MEMS transducer 100 and an electronic circuit 102.
The MEMS transducer 100 is shown as being formed on a separate integrated circuit to the electronic circuit 102, the two being electrically connected using, for example, bond wires 112, 124. The MEMS transducer 100 comprises a MEMS capacitor CMEMS having first and second plates 118, 120 that are respectively connected to first and second bond pads 114, 122.
The electronic circuit 102 comprises a charge pump 104, a diode 106, a reservoir capacitor (CRes) 108, an amplifier 128, a bias circuit 131, third, fourth, and fifth bond pads 110, 126 and 130, and an optional digital-to-analogue converter (DAC) 132 with an associated sixth bond pad 134.
The following now describes the basic operation of the MEMS device.
The charge pump 104 receives a supply voltage VDD and a first reference voltage VREF1 and outputs an output voltage VDD* (that is greater than the supply voltage VDD). The output voltage VDD* charges up the reservoir capacitor 108, via the diode 106, to a first bias voltage Vb. The reservoir capacitor 108 supplies a relatively stable, i.e. clean, voltage Vb, via the bond pad 110, the bond wire 112 and the bond pad 114, so as to bias the first plate 118 of the MEMS capacitor CMEMS.
The MEMS capacitor CMEMS outputs, via the second bond pad 122, an analogue voltage signal in response to a sound pressure wave.
The amplifier 128 receives, via the bond pad 122, the bond wire 124 and the bond pad 126 the analogue voltage signal from the MEMS capacitor CMEMS, and amplifies the analogue voltage signal. The amplified analogue signal, which may be a current or a voltage depending upon the type of amplifier used, is then output, for further processing, via the fifth bond pad 130. Alternatively, the electronic circuitry 102 may comprise a DAC 132, in which case, the amplified analogue signal is output, via the sixth bond pad 134, as a digital signal. The digital signal may be output instead of, or in addition to, the amplified analogue signal. The amplifier also receives from the bias circuit 131, a second bias voltage VREF2 via a bias impedance (not illustrated). The second bias voltage VREF2 also biases the second plate 120 of the MEMS capacitor CMEMS.
As can be seen in FIG. 1, a transducer (CMEMS) can be fabricated on a separate integrated circuit to its associated electronic circuitry. The separate integrated circuits (100, 102) can either be packaged separately, or mounted on a common substrate within the same package. When the transducer and associated electronic circuitry are formed on separate integrated circuits, interconnecting elements such as bond wires (for example bond wires 112, 124 shown in FIG. 1), or studs, bumps etc. are used to electrically interconnect the separate integrated circuits 100, 102. It should be noted that a transducer and its associated electronic circuitry can also be fabricated on the same integrated circuit, i.e. a fully integrated solution. The present invention is also applicable and/or adaptable to such fully integrated solutions.
As with conventional silicon technology, MEMS technology allows much of the manufacturing process to be performed on many devices at once, on a whole wafer containing thousands of devices, or even a batch of dozens of wafers. This fundamentally reduces production cost. Wafer-scale packaging techniques may also be used with similar benefits.
However, the production process contains many steps, not only the silicon-level processing steps, but also later steps, for example placing the transducer on a common underlying substrate with the amplifier and biasing electronics, adding bond wires between the transducer and the electronics and from the electronics to terminals on the substrate, covering the assembly with protective material, and adding a case to cover the assembly. At each stage, processing errors may occur, or random defects may degrade the device, so it is desirable to be able to test the functionality of the sub-components and their interconnections as soon as possible in the manufacturing process, to avoid wasting the cost of materials and processing devices that will be rejected at final test.
It is not straightforward to apply conventional wafer-test techniques to capacitive transducers. For example, in the case of a microphone application, it is impractical to apply a controlled acoustic stimulus to each MEMS die on a wafer. Also, because of the very low capacitance of the sensor (possibly less than 1 pf), and hence the small input capacitance necessary of the amplifier electronics, there may be little or no electrostatic discharge (ESD) protection on the amplifier input, so these inputs are liable to damage if probed directly during testing. Also the amplifier performance may be altered by the parasitic capacitance of the probes being applied to its input. Therefore, it is desirable to be able to test the functionality, electrical continuity or performance of the device with neither an acoustic stimulus nor direct electrical contact to sensitive circuit nodes.
Furthermore, the need for low cost and high volume means that the test time should be as short as possible, so preferably tests for gross failure modes should be performed and samples failing these functional tests should be removed from test before any time-consuming precision tests are carried out. Once a production line is characterised and under Statistical Process Control, a largely functional test may be adequate to obtain a low defect rate. However, even on a mature process there is the need for occasional auditing and re-characterisation to allow yield optimisation or to help diagnose the causes of any reduction in yield. It is useful to be able to access different nodes in any circuitry to provide clues to any yield sensitivity, for example to localise a problem to a particular part of the circuitry.
However one problem in fully testing finished devices is that because of size constraints on the overall package size, there may only be a very small number of external connections to the transducer/circuit assembly, possibly as few as three (ground, supply, and output). This makes it difficult to access internal nodes in a circuit, so as to apply electrical signals to these nodes, for such test and diagnostic purposes.
The present invention seeks to provide a method of testing a high input impedance transducer, such as a capacitive transducer for example that may be realised as a MEMS transducer, and/or its associated electronics, that allows test stimuli to be applied without a physical stimulus (e.g. pressure stimulus) or direct external electrical connection to critical nodes (e.g. probing sensitive nodes), while not impacting performance nor requiring complex additional circuitry.